The invention relates generally to pneumatically powered hand tools and more specifically to a reversible double-throw air motor for use with such tools.
Various pneumatic impulse tools, such as impact wrenches, are powered by reversible rotary vane pneumatic motors. Such motors are required to have a large stall torque in both forward and reverse directions. It is advantageous for such motors to be relatively small in size, since they are generally hand-held by an operator.
Most previously known reversible air motors are changed from forward to reverse operation by rerouting the inlet (pressure) and outlet (exhaust) paths at a location remote from the motor package, such as by shuttle spool valves or rotary valves. Such reversing arrangements take up valuable space, making the tool larger, complicate the construction in terms of adding parts and requiring additional labor for assembly, thus increasing the manufacturing cost, and creating tortuous air flow paths, thus reducing efficiency.
U.S. Pat. No. 4,822,264 to Kettner discloses a rotary vane air motor/reversal package having five main parts--a housing; a cylinder member; a rotor assembly; a distributor; and a valve plate, each of relatively complicated design and calling for precision manufacture to minimize leaks. In the Kettner device, the supply and exhaust passages leading to and from the cylinder chambers are reversed by changing the rotational position of a rotary valve plate that is positioned between a fixed distributor mounted within the motor casing on a rear side of the valve plate and a fixed cylinder casing on the front side of the valve plate. Although the design of Kettner's motor improves on some prior art reversible rotary vane motors in terms of size, it has some shortcomings. The distributor has two pressure ports located diametrically opposite each other, each of which is flanked on either side by an exhaust port. The exhaust ports are located very close to the pressure ports, thus presenting an opportunity for blowby of pressure air at the interface between the distributor and the valve plate. That possibility is exacerbated by the fact that the rotatable valve plate interfaces on opposite sides with fixed members with sliding fits. Thus, small tolerance variations can lead to large leaks and reduced efficiency. In addition, the location of the rotary valve plate, upstream from the motor's cylinder, requires that the actuator for the rotary valve plate (i.e., the part the user touches to switch between forward and reverse) is physically located rearward of the motor's cylinder. From an ergonomic perspective, this placement of the actuator is somewhat undesirable, as a location closer to the front end of the device would be more easily manipulated by the user under normal gripping circumstances. Further, the position of the valve plate is maintained by a spring/ball detent; avoiding the risk of an unintended rotation of the valve plate during handling of a tool equipped with the motor requires that the detent be quite strong which detracts from a desirable facility of reversal by the user. If the valve plate is rotated inadvertently from a desired position during handling, there is no assurance that it will be moved to the proper position during operation of the tool, and the motor performance may be compromised, resulting in a defective operation, such as a low torque on a fastener.
Thus, there remains a need for an improved design of a reversible double-throw air motor. Such a motor should allow for easy use and low production costs.